Examples of an arc welding apparatus are a pulse arc welding and short-circuit transferring arc welding apparatus. Some pulse arc welding apparatus makes use of a train of pulse current groups each of which includes a plurality of pulses. In the former pulse arc welding apparatus, a pulse arc current is produced between a consumable welding wire electrode (referred to merely as a wire electrode hereinafter) and a base metal, and heat generated by a pulse arc discharge produced by the pulse arc current causes both the base metal and the wire electrode to be melted as well as causes the melted wire electrode to be transferred onto the base metal by virtue of electromagnetic pinch force developed through pulse arc discharge, thereby welding the base metal. In the latter pulse arc welding apparatus, a plurality of pulse currents form a pulse current waveform. Each of pulses is further divided into a plurality of pulses to form a discharge current waveforms that are periodically repeated. This division of pulse current waveform causes the upward magnetic force resulted from pulse arc discharge of the wire electrodes to be intermittent, thereby reducing the force tending to lift a molten droplet produced on the tip of the wire electrode.
The short circuiting arc welding apparatus is the one in which the base metal and wire electrode are melted by the heat generated through arc discharge which in turn is generated when short-circuiting is repeated alternately with arc to produce arc currents that flows between the base metal and wire electrode, and then the base metal and electrode are short-circuited such that molten droplet produced on the tip of the wire electrode is transferred onto the base metal for welding.
The former pulse arc welding apparatus will now be described specifically. FIG. 53a shows a prior art pulse arc welding apparatus disclosed by Published Unexamined Japanese Patent Application No. 57-19177. In FIG. 53a, reference numeral 92 depicts a d-c power source, 93 is a switching element in the form of a power transistor that controls current in chopper action in which the current supplied from the d-c power source 92 is switched on and off to form a pulse-like current waveform. The apparatus further includes a welding torch 51 that serves as an arc load, a wire electrode 52 which is a heat melted metal in the form of a wire supplied from a wire reel, an arc discharge 53, and base metals 46 and 54. The reference 93 is a base current supplying device for supplying a continuous base current such that the arc exists continuously between the pulses. The reference numeral 94 denotes a control circuit for controlling the switching element (95) to maintain the pulse frequency and pulse width of the pulse current at a predetermined value, and the numeral 6 is a current detector for detecting a current i.
The operation of the pulse arc welding apparatus will now be described.
In general, a pulse arc welding apparatus is advantageous in that the tip end of the wire electrode 52 is melted by the pulse arc current and the melted tip of wire electrode is transferred onto the base metal by the electromagnetic pinch force even if the average current thereof is smaller than that of the d-c arc welding apparatus. Thus, the average current is lower in the pulse arc welding apparatus than in the d-c arc welding apparatus, which enables welding of thinner base metals. In addition, "spatter" that occurs during welding may be eliminated due to the fact that the tip of electrode can be separated in air from the wire with the aid of electromagnetic pinch force.
FIG. 54a shows an example of a pulse arc current waveform, produced when the wire electrode is a soft steel of 1.2 mm diameter and the shielding gas is a mixture of argon plus 20% CO.sub.2.
The short circuiting arc welding apparatus will now be described specifically. FIG. 53b shows a construction of a conventional short circuiting welding apparatus disclosed by Japanese Patent Publication No. 62-54585.
In FIG. 53b, the numeral 7 is a voltage detector for detecting a voltage V across the wire electrode 52 and the base metal 54 to be welded, the numeral 94a is a first decision-making device which compares the detection voltage V detected by the voltage detector 7 with a voltage Vo (a voltage during short-circuiting or a voltage just before a short-circuiting occurs) indicative of short circuiting condition to thereby decide the short circuiting where a switch command circuit 94c is supplied with a command signal when V.ltoreq.Vo so as to shift the switching element 95 to ON position. The numeral 94b is a second decision-making device which compares the detection voltage V detected by the voltage detector 7 with a voltage Va indicative of arc reproduction to thereby decide the arc reproduction where a switch command circuit 94c is supplied with a command signal to shift the switching element 95 to OFF position. FIG. 54b illustrates the current waveform of this welding machine.
The operation of the welding machine will now be described. The tip of the wire 52 is short-circuited to the base metal 54 with both the d-c power source circuit 92 and the base current supplying device 93 turned on (not shown). Since the detection voltage V detected by the voltage detector 7 is lower than the voltage Vo representative of the short-circuited condition (V.ltoreq.Vo), the first decision making device 94a becomes operative to output an ON signal to the switch command circuit 94c which in turn outputs a trigger signal to close the switching element 95 so as to cause the d-c power source circuit 92 to supply a current. This current flows till the wire electrode 52 is burned to cut off and an arc is produced while the detection voltage V detected by the voltage detector 7 increases from a short-circuit voltage to an arc voltage. Then, the first decision making device 94a is stopped to operate. Where the detection voltage V increases higher than the voltage Va indicative of the arc reproduction (V.gtoreq.Va), the second decision making device comes into operation to shut off the trigger signal to the switch command circuit 94c so as to open the switching element 95. Thus, the current is decreased by a reactor 1b, thereby the current is supplied only by the base current supplying device.
In this manner, the wire electrode and the base metal are heated to melt during the first arc period B, the welding wire 52 is supplied by a supply-motor to the torch 51, the melted tip of the wire 52 is short-circuited to the base metal which causes the switching element 95 to close. This allows the d-c power source circuit 92 to supply current and the transfer of the wire electrode 52 onto the base metal 54 is completed.
Repeating the above-described process will result in a waveform shown in FIG. 54b which permits a stable welding operation
For good welding result in the aforementioned pulse arc welding, it is necessary to prevent spatter of the welded materials produced during welding and undercut which is a defect in shape of weld bead and to maintain generally uniform size of molten droplet which is separated from the wire electrode. The wire electrode must not touch (short-circuit) the base metal so that the spatter is eliminated. The arc length should be short so that the undercut is prevented. To meet these two requirements, it is a critical importance that the separated molten droplet is transferred in the form of fine particles onto the base metal to be welded. (Spray-transfer) The uniform size of separated molten electrode can be obtained by repeating the same shape of pulse to produce a pulse arc current waveform.
In a shielding gas of a mixture of argon and 20% CO.sub.2, the size of arc is sufficiently large as compared to the molten droplet of wire electrode, thus the periodic repetition of simple pulses shown in FIG. 54a will cause the molten droplet to regularly separate from the electrode, which promises good quality of welding. On the other hand, in a shielding gas of 100% CO.sub.2, the size of arc is rather small as compared to the molten droplet of wire electrode, thus the simple pulses shown in FIG. 54a will cause the phenomena shown in FIGS. 55a and 55b resulting in poor welding quality. Setting a lower base current I.sub.B as shown in FIG. 55b and a wider pulse width .tau. will cause the magnetic force F due to the pulse current to act upwardly so that the shape of molten droplet 52a of the tip of the wire electrode 52 is lifted from Po to Pb1. Then, the molten droplet 52a can be separated by the pulse current as depicted by Pb1. The separated molten droplet 52a goes into a high speed rotation not dropping onto the base metal but scattering as a spatter or again being deposited on the wire electrode 52 as depicted by Pb2'.
A pulse welding apparatus shown in FIG. 56 is one of the apparatus in which pulse arc welding is carried out in a shielding gas of 100% CO.sub.2.
The pulse arc current waveform used in this apparatus is formed of a plurality of pulse currents as shown in FIG. 57. Thus, the discharge current waveform is of a plurality of pulse currents which is repeated periodically. That is, each pulse is divided into a plurality of pulses. The division of the pulse current waveform causes the upward magnetic force due to pulse arc discharge on the wire electrode to be intermittent, thus reducing the force tending to lift the molten droplet produced on the tip of the wire electrode. As a result, the molten droplet may easily be separated from the electrode before it becomes a large size not only in an argon-dominated shielding gas but also in a shielding gas of CO.sub.2 gas.
The transfer phenomenon of the molten droplet will be described.
As shown in FIG. 57, periodically running a group of pulse arc current of a predetermined pulse width .tau. and a period C.sub.A causes the molten droplet produced on the wire electrode to go through "growth of molten droplet" alternately with "the separation of the molten droplet" in a regular manner as shown in FIG. 57 in synchronism with the pulses. In other words, the sufficient amount of a molten droplet on the wire electrode produced at the beginning of a group of pulses are constricted, the constriction of the molten droplet is further developed to separate the molten droplet from the electrode, then after the molten droplet has separated from the electrode, another molten droplet will grow on the tip of the wire electrode due to pulses, while being lifted upwardly. During the subsequent base period, the lifted molten droplet on the tip of wire will hang and hold the shape of the molten droplet till a group of pulses are supplied.
A pulse arc welding apparatus of the aforementioned arc welding apparatus in which the arc current is a pulse current group formed of a plurality of pulses, will now be described with reference to FIG. 56 and 57. FIG. 56 is a general block diagram of a prior art pulse arc welding apparatus.
An arc welding power supply for supplying current comprises the following elements. Reference numeral 1 is an invertor drive circuit which converts a three phase a-c voltage into a predetermined frequency to output it to a transformer 3. Reference numeral 2 is an invertor drive circuit for driving the invertor circuit 1. Reference numerals 4A and 4B are diodes for rectifying the transformed invertor-output to produce an arc current consisting of pulse currents. Reference numeral 51 is a welding torch, numeral 52 is a wire electrode supplied between supply rollers from the wire reel toward the base metal to be welded 54, the numeral 7 is a voltage detector for detecting an arc voltage, and the numeral 6 is a current detector for detecting the arc current. Reference numeral 9 is a wire-supplying rate setting device for setting the wire-supplying rate, numeral 10 is a wire-supplying device for supplying the wire electrode 52 to the base metal 54 to be welded, numeral 11 an average voltage setting device for setting an average arc voltage. Reference numeral 8 is a pulse current waveform control circuit for setting a group of pulse currents to output them as an arc current. The control circuit 8 includes a pulse waveform shaper 81, a pulse-group period C.sub.B setting device 82, a pulse group duration X setting device 83, a pulse group waveform setting device 84, a pulse width .tau. setting device 85, a pulse period C.sub.A setting device 86, an adder 89 for adding the produced pulse current group to the base current supplied by a base current outputting device 88, a comparator 89 for comparing the detected arc current with the pulse current group output, and a pulse current outputting device 81a.
FIG. 57 illustrates how the molten droplet undergoes its states when welding is performed by the use of the aforementioned pulse arc current group waveform. In the figure, I.sub.P is a pulse peak current, .tau. is a pulse width, T.sub.A is a time interval between pulses within a group X of pulse currents, I.sub.B is a base current, T.sub.B is a repetitive period between the groups X of pulse currents, and C.sub.B is a repetitive period of the group X of pulse currents.
The operation of the prior art apparatus will now be described.
First of all, the pulse waveform shaper 81 receives a pulse-group waveform signal from the pulse group waveform setting device 84, a pulse width .tau. signal from the pulse width .tau. setting device 85, and a pulse period C.sub.A signal from the pulse period C.sub.A setting device 86, respectively. The pulse-group period C.sub.B setting device 82 sends out a pulse-group period C.sub.B signal to the pulse waveform shaper 81 and the pulse-group duration X setting device 83 a pulse-group duration X signal. The pulse waveform shaper 81 shapes the pulse-group signal having a specific pulse group waveform and a pulse period C.sub.A into an intermittent-group-of-pulses waveform as shown in FIG. 57 on the basis of the aforementioned pulse-group period C.sub.B signal and pulse-group duration X signal, which is then further shaped by the base current outputting device 88 into a waveform in which the base current I.sub.B signal is superposed (FIG. 57). The thus shaped pulse current-group signal and the current signal detected by the current detector 6 are inputted into the comparator 89 so as to transmit the invertor drive signal in accordance with the magnitude relation between the pulse current group signal and the detection current signal from the invertor drive circuit 2 to the invertor circuit 1, thereby driving the invertor. By driving the invertor, a group of pulse arc currents as shown in FIG. 57 are supplied to the weld zone.
The arc load 5 is being continuously supplied with the wire electrode 52 by a motor (not shown) as well as the group of pulse arc currents. Thus, the group of pulse currents creates a pulse arc discharge 53 between the wire 53 and the base metal 54 to be welded. This pulse arc discharge 53 causes both the tip of the wire electrode 52 and the base metal 54 to melt. The melted tip end of wire electrode 52 falls successively onto the molten pool of the base metal 54, thus performing welding. Therefore, the wire electrode 52 is consumed continuously. The above-described motor runs to continuously supply the welding torch 51 with the wire electrode 52 to replenish the electrode being consumed.
The high frequency characteristics of the above-described pulse arc current waveform I.sub.P will now be described with reference to FIG. 57. The narrow pulse width .tau. of each pulse and the intermittent pulse current within the pulse current-group X cause variations of electromagnetic force in accordance with the application of pulse currents. Then, most of the force acting on the molten droplet 52a produced on the tip of wire electrode 52 is due to the electromagnetic force created by the pulse peak current I.sub.P while the pulse current is flowing. When the pulse current is not flowing, forces such as reaction against the electromagnetic force during energization by pulse current, the surface tension of molten droplet, or weight are much larger than the electromagnetic force due to the base current. These forces act as a pinch force on the molten droplet 52a. Thus, this means that the molten droplet 52a produced on the tip of the wire electrode 52 vibrates at the frequency of the pulse current-group X. The vibration of the molten droplet 52a causes an early "constriction B" at the boundary of the wire electrode and the molten droplet which conventionally was difficult to be constricted at pulse peak current I.sub.P. This facilitates the separation of the molten droplet from the electrode.
In welding, forming the molten droplet into fine particles with the aid of the pulse current-group X and having the fine particles transferred onto the base metal in a regular manner, results in a uniform bead. Thus, it is necessary to repeat at a predetermined period C.sub.B the pulse current-group X which is formed of a plurality of pulse currents having a predetermined pulse interval T.sub.A and a pulse width .tau..
When arc welding is performed with the arc-creating wire electrode moving in a predetermined direction above the base metal, the distribution of magnetic field formed in welding-area varies in accordance with the path through which the current flows, i.e., from welding torch to arc, from arc to weldment. In other words, the distribution of magnetic field which depends on the shape of welded joint and the location of ground point, will vary from case to case. A magnetic force produced by an arc current in the distributed magnetic field will act on the arc to cause a magnetic blow phenomenon such that the arc is tilted against the base metal. As depicted by (A-1) to (C-1), (A-3) to (C-3) in FIG. 58, the magnetic field blow phenomenon causes the arc length to be longer and causes the regular separation of molten droplet to be difficult for the reason that the molten droplet is lifted by the deflected arc. In order to solve this kind of problem, and to make the instantaneous arc length equal to an aimed arc length according to a target arc length signal. Thus, the arc current through the wire electrode is decreased to reduce melting capacity as well as the magnetic force which is proportional to the square of the arc current I is reduced to alleviate the lift of the molten droplet due to magnetic field blow.
As for short circuiting arc welding, as depicted by (S1a) to (S3a) shown in FIG. 59, when the molten droplet grown on the tip of the wire electrode is pushed up by the arc deflection due to magnetic blow, the timing at which the molten droplet is short-circuited varies, disturbing the repetition period of short-circuiting and arc.
In order that the arc current is controlled to retard the lift of molten droplet due to magnetic blow, the instantaneous arc lengths need to be controlled so as to be the same as the target arc length signal. This is achieved by reducing the arc current through the wire electrode to decrease the melting capacity thereof and to weaken the magnetic force proportional to the square of arc current I, thereby retarding the lift of the molten droplet due to magnetic blow.
However, when the pulse arc welding is to be performed in the shielding gas of 100% CO.sub.2 gas, a small pulse peak current I.sub.P will cause the molten droplet produced at the tip of wire electrode to be lifted by the pulses, the molten droplet not being able to leave the electrode till it is too large. As a result, the molten droplet on the tip of wire electrode becomes large to be short-circuited to the weld zone. This causes a large amount of spatter to scatter around the welding site during welding process or causes the undercut, a defective bead. Additionally, higher pulse peak currents I.sub.P leads to bulky size of the power supply of the apparatus and more weight, resulting in an abrupt increase in costs.
In order to solve this problem, Japanese Patent Application No. 62-309388 and No. 63-265083 have been filed, in which one pulse current waveform is divided into a group of pulse currents which have more than one pulse width and are disposed at more than pulse intervals; this group of pulses is repeated with a period therebetween and a continuous base current is superposed thereto to form a discharge current waveform; thus, the lift exerted on the molten droplet produced on the tip of wire electrode is reduced to convert the molten droplet, which is to be transferred onto the weld zone, into fine particles, thereby allowing the regular transfer of the molten droplets.
However, for this pulse welding apparatus, if the molten droplet is late to leave the electrode flue to variations of welding conditions and external disturbances, the molten droplet on the tip of wire electrode cannot grow sufficiently during the next period causing difficulty for the molten droplet to leave the electrode or becomes irregular, the quality of welding being impaired. Thus, the factors of variations of the welding conditions need to be controlled and measures for preventing external disturbances are necessary.
It is also necessary to vary the speed, at which the wire is supplied, in accordance with the base metal (metal to be welded). In which case, the pulse current group waveform is also required to be varied when the wire-supplying speed is varied. Since only the period of the pulse currents group is varied in accordance with the wire-supplying speed, even when the wire-supplying speed at which the wire electrode is supplied is varied below a certain value, or the wire-supplying speed or the pulse group period is varied, the interval between the respective pulses within a pulse group remains the same. Therefore, if the pulse group period becomes longer when the wire-supplying speed decreases, the rate of growth of molten droplet due to the pulse current group during the pulse group period becomes higher than the wire-supplying speed, causing the arc-length between the wire electrode and the base metal to be longer than an allowable value. This causes the base metal itself to be melted, resulting in too wide a melted width of the base metal. The wide melted width contributes to undercut; welding quality is impaired.
Likewise, the average voltages of pulse current group waveform higher than a predetermined value cause the peak value of the pulse group current waveform during the pulse group period to be higher. This results in higher current values per unit time and higher rate of growth of the molten droplet produced by the pulse current group, and the arc length between the wire electrode and the work tends to be longer than the predetermined value. As a result, the molten droplet width of the base metal, i.e., a portion of the base metal melted by the arc becomes too wide, being a source of undercut and impaired welding quality.
Additionally, the amount of melted electrode after the molten droplet leaves the wire electrode is greatly different from that before the molten droplet leaves the wire. Thus, the magnetic force produced by the pulses is not large enough to lift the molten droplet up before the molten droplet leaves the electrode, but is large enough to lift a small amount of molten droplet left on the tip of wire after the molten droplet has left the wire. While such a difference exists in the molten droplet-lifting effect, the aforementioned pulse welding apparatus has a fixed pulse interval, fixed pulse width, and fixed pulse period within the pulse group throughout the respective processes during welding, i.e., from growth of molten droplet to separation of the molten droplet. Thus, the molten droplet left on the wire tends to easily be lifted after the previous molten droplet has left the electrode as well as the arc length between the wire electrode and the base metal tends to be longer than the allowable value. This causes the base metal itself to be melted, resulting in too wide a melted width of the base metal. The wide melted width contributes to the undercut, causing poor welding quality.
In other words, such a lift of the molten droplet causes the quality of welding bead to be affected by changes in welding conditions or by external disturbances, as well as causes period required for the globule to leave the electrode to vary. This leads to the difficulty in obtaining uniform welding result.
With the pulse interval, pulse width, and pulse period, all being fixed, the melted portion of the wire electrode after the globule has left the electrode tends to be lifted by the pulses, thus the arc length tending to vary. Therefore, the period at which the globule leaves the electrode varies causing a difficulty in maintaining regular separation of globule to obtain uniform welding bead.
In order to hold the instantaneous arc length to a value corresponding to the target arc length, if the lift of globule due to the magnetic blow is to be retarded by decreasing the arc current through the wire electrode to decrease melting capacity of the electrode as well as the electromagnetic force proportional to the square of arc current I, the decreased arc current in turn retards the growth of globule on the tip of wire electrode, resulting in the delay of separation of molten droplet from the tip of wire electrode and disturbing the regular separation of molten droplet to impair the quality of welding bead.
According to the short circuiting arc welding apparatus shown in FIG. 53b, in order to hold the instantaneous arc length to the target arc length, if the lift of molten droplet due to the magnetic blow is to be retarded by decreasing the arc current through the wire electrode to decrease melting capacity of the electrode as well as the electromagnetic force proportional to the square of arc current I, the decreased arc current retards the growth of molten droplet on the tip of wire electrode, the transfer of molten droplet from the electrode to the base metal to be welded is not carried out by short-circuiting during the next short-circuiting period, causing the spatter to occur and eventually being difficult to obtain good welding bead.
If the molten droplet is late to leave the electrode due to variations of welding conditions and external disturbances, the molten droplet on the tip of wire electrode cannot grow sufficiently during the next period causing difficulty for the molten droplet to leave the electrode or becomes irregular to impair the quality of welding. Thus, the control of the factors of variations in welding conditions is necessary and adequate measure is required to prevent external disturbances. This leads to a complicated welding control.
Objects of the present invention are as follows:
An object of the present invention is to retard the variation of the time required for the molten droplet to leave the electrode to perform high quality welding.
An object of the present invention is to provide a pulse welding apparatus in which even if the wire-supplying speed varies, the arc length can be controlled below an allowable value where no undercut is developed, thereby enabling good welding.
An object of the present invention is to provide a pulse welding apparatus in which even if welding is to be carried out at low wire-supplying speeds and with high average voltages, the arc length can be maintained below an allowable value where no undercut is resulted, thereby enabling good welding.
An object of the present invention is to provide a pulse welding apparatus in which even if the welding is to be performed with long pulse group periods, the arc length can be controlled below an allowable value where no undercut is resulted, thereby enabling good welding.
An object of the present invention is to provide a pulse welding apparatus in which even if the molten droplet leaves the electrode at different separation timing, the arc length can be controlled below an allowable value where no undercut is resulted, as well as uniform welding beads are resulted in.
An object of the present invention is to provide a pulse welding apparatus in which the separation period of molten droplet is prevented from varying due to long arc length after the molten droplet has left the electrode to thereby carry out regular welding so as to obtain uniform welding beads.
An object of the invention is to provide an arc welding apparatus in which the separation of molten droplet is prevented from being irregular due to magnetic blow phenomena and the melting capacity of molten droplet at each period is ensured to make separation period regular for good arc welding.
An object of the invention is to provide an arc welding apparatus in which short-circuiting of molten droplet is prevented from being irregular due to magnetic blow phenomenon as well as the melting capacity of molten droplet during arc period is ensured to implement a regular short-circuiting period for good short-circuiting welding.
An object of the invention is to provide an arc welding apparatus in which even if the molten droplets vary in separation timing, good welding can still be carried out in accordance with the varying separation timing of the molten droplet.